Drying kilns are employed to dry and season a variety of products, such as fruits and vegetables, meats and fish (often with the additional presence of smoke), various clay and ceramic products, and particularly lumber. In the lumber industry drying kilns are employed to season and to "dry" both green wood and partially air dried wood. The lumber industry uses the word "dry" to mean bringing the wood to the appropriate degree or range of moisture retained in the wood which is optimal both for the particular specie of wood and the lumber dimensions being "dried". Modern lumber industry drying kilns are actually sophisticated, controlled humidity environments.
Lumber is conventionally piled into pelletized stacks for drying. Each stack is formed by alternating layers of green lumber laid side-to-side with a layer of "stickers", each typically 5/8" by 5/8" and spaced apart by 12"-36", until a stack of the desired height is obtained. Stacks may also be stacked one atop another. Typically, an elongated structure is employed for the lumber kiln, currently between 60' and 120" long with double cribs, or tracks, holding 16,000 board feet per 10' of kiln length, in which one or more stacks of green lumber are arranged so that a flow of drying air may be directed over and through those stacks and thereby dry the lumber.
Typically, fans are employed with conventionally arranged ducting to either draw or push air through a stack in from one stack surface and out through an opposite stuck surface. Modern lumber kilns commonly recirculate the drying air back to the stack through further conventionally arranged ducting and humidity and temperature control apparatus. State of the art systems employ both temperature and humidity controls associated with fans and ducting to control the humidity and temperature of the drying air and to recirculate it through the stacks of green lumber until such time as the lumber has reached the desired degree of dryness.
The problem of directing the flow of drying air efficiently through, and not over, a stack of green lumber has long begged a solution. Woolhouse (U.S. Pat. No. 1,919,646) discloses a curtain means 16 employed in such an attempt. One device disclosed in U.S. Pat. No. 3,196,554 to Smith discloses a series of supposedly inflatable baffles for non-heated pre-drying to serve much the same purpose as a series of Woolhouses's curtains for non-heated predrying. However, no known methods effectively baffle substantially all of the drying airflow to prevent it from flowing around the product stack so that drying air flow is directed to flow only through the product stack. Also, there are no known methods for baffling substantially all of one or more of the stack surfaces to constrain drying airflow from the baffled surface and direct it through the stack.
Drying air which by-passes the product stack to flow around the top, sides, ends, and sometimes the bottom of the stack in conventional drying kilns has a higher velocity then the air which flows through the product stack, and usually has a higher temperature as well. This by-passing air flowing over the top, sides, ends, and sometimes the bottom of the stack results in the top, ends, sides, and sometimes the bottom of the stack drying faster then the rest of the stack. This can, and often does, result in overdried and degraded product on the stack surfaces which are exposed to the higher velocity and higher temperature by-passing airflow. Current methods of regulating airflow require considerable power consumption by fans since substantial amounts of drying airflow escape around the sides of the stacks of lumber, so that a greater airflow must be generated by the fans then is actually used to dry the product in the stack, thus requiring wasted and power consuming recirculation of extra air by the fans. An extra 75 fan horsepower in a kiln at 5 cents per kilowatthour costs the kiln owner $27,000 per year in electricity.
Known methods of baffling also require either uniform loads within a particular kiln or at least loads which do not vary significantly in gross surface dimension of the load over the surface of any particular side of the load. The baffles disclosed by Smith permit airflow to escape in a somewhat winding path around the edges of the various interwoven baffles therein disclosed, even if a Smith baffle could be made to inflate as disclosed and to effectively seal against the load surface at all.
It is essential that this drying operation be cost effective, particularly in times when the lumber industries is troubled because every cubic foot of air bypassing the product stack is a waste of energy and money. Thus it is necessary both to employ a standard sized kiln which can accommodate various sizes of stacked green loads of lumber and to use only the fan horsepower needed to accomplish the drying without wasted airflow.
An effective baffling system is needed which prevents excessive air flow across the entirety of any surface, especially the top surface, of a stack of lumber and which can block the flow of air past the ends of lumber in a stack, while allowing for large variations in gross stack surface dimensions and in distances between stacks and between a stack surface and a kiln wall, particularly the end wall.